Description (adapted from the application): Two groups of patients with chronic fatigue syndrome have been identified. One group has a putative neurological cause for the illness and a second group a putative cardiovascular cause. The index of neurologic dysfunction is the presence of abnormalities on cognitive testing and the index for cardiovascular dysfunction is the presence of a low stroke volume. The applicants acknowledge the possibility of overlap of the two conditions as well as the presence of a group exhibiting neither abnormality. Thus, in any one project, 4 groups of patients with chronic fatigue syndrome will be tested. These will be compared to a control group of sedentary individuals. Five studies are planned. Study 1 assesses the constitution of spinal fluid for markers of inflammation as well as for 5-HIAA, MHPG and substance P. Study 2 proposes to evaluate changes in the plasma prolactin, ACTH and cortisol in response to the intravenous infusion of a serotonin uptake blocker. Study 3 proposes to use xenon CT scanning to assess cerebral blood flow. Study 4 proposes to use radionuclide ventriculography to assess cardiac function at rest and during emotional, orthostatic and physical stress. Study 5 proposes to lower body negative pressure to provide a graded assessment of orthostatic tolerance.